1. Field of the Invention
The present invention relates to a method of controlling a fuser unit of an image-forming apparatus, especially at the warm-up stage of the apparatus.
2. Description of the Related Arts
In an electronics image-forming apparatus, such as a copier, printer, or facsimile machine, a toner image formed on a photoconductive drum is transferred to a sheet medium and fixed thereon by a fuser unit.
In FIGS. 7(A) and 7(B), which illustrates a principle of the toner fixing process, a fuser unit 14 comprises a pair of a heat roller 10 and a backup roller 12. The heat roller 10 is made of a heat-conductive material, such as an aluminum tube coated with a layer of a heat-durable resin and is fitted with an internal built-in heater. The backup roller 12 is made of an elastomeric material, such as a silicone rubber, and is pressed against the surface of the heat roller 10 to be frictionally driven by the rotation of the heat roller 10, which is in turn driven by a main motor of the apparatus. Accordingly, the rollers 10 and 12 rotate together during the printing process and nip a sheet medium 16 therebetween to heat-fix a toner image 18 carried on the sheet medium 16.
To achieve a favorable printing quality, it is important to initialize the mechanical and electrostatic conditions of the machine before starting the printing process. In the initialization, a single main motor of the apparatus is rotated to thereby drive all rotating elements of the machine including the heat roller 10. At the same time, the built-in heater of the heat roller 10 is energized to elevate the temperature thereof.
The initialization lasts for only a short period, because it imposes an unfavorable stress on process elements such as a photoconductive drum or a developer, and thus shortens the life span thereof. Therefore, the main motor is stopped immediately after the initialization period is completed, and thus all of the rotating elements in the apparatus become stationary. The built-in heater of the heat roller 10, however, is still energized while a surface temperature of the roller 10 is monitored by a sensor, and when the monitored temperature reaches a predetermined value, it is determined that the warm-up stage is completed and the apparatus is ready to start the printing process.
Nevertheless, a problem arises in the abovementioned steps in that the heater of the heat roller 10 is energized while the roller 10 is stationary after the initialization has been completed. As shown in FIG. 7(A), a temperature distribution of the heat roller 10 and the backup roller 12 in this case is such that a whole periphery of the heat roller 10 including the topmost point HT 10 and the bottommost point HB 10 is equally heated by the built-in heater, whereas in the backup roller 12, although a region in the vicinity of the topmost point HT 12 is heated to substantially the same level as the heat roller 10, by heat conduction from the heat roller 10, the lower region of the backup roller 12 remains at a lower temperature because of a relatively poor heat conductivity of the silicone rubber forming the same, whereby a temperature gradient is formed through the backup roller 12 from the topmost point HT 12 to the bottommost point HB 12. Accordingly, if the printing process is started immediately after the surface temperature of the heat roller 10 has reached the predetermined value, the heat stored in the body of the heat roller 10 is transferred to the lower temperature region of the backup roller 12 in the vicinity of the point HB 12, every time the point HB 12 is in contact with the heat roller 10, as shown in FIG. 7(B), and this causes the surface temperature of the heat roller 10 to drop below the predetermined lower limit value for fixing the toner 18 on the sheet medium 16. This phenomenon is particularly serious when the apparatus is non-operative for a long time in low ambient temperature conditions. The temperature transition of each of the rollers 10 and 12 during the initialization and warm-up stage is illustrated in FIG. 8, in which the surface temperature of the heat roller 10 becomes lower than the lower limit for a period t, even after the predetermined temperature has been once obtained.
To solve the above problem, as shown in FIG. 9, Japanese Examined Patent Publication (Kokoku) No. 61-31462 (corresponding to U.S. Patent No. 4,385,826) proposed that the energization of a heater in a heat roller be started while the roller is stationary (step 900), this energization be continued for a predetermined period (step 902), and then the main motor driven to rotate the heat roller together with a backup roller until the surfaces of both rollers are uniformly and sufficiently heated (904).
According to this method, however, the mechanical/electrostatic stresses stored in the process elements are larger than in the usual case because the process elements must be additionally driven together with the heat roller for a longer period. Further, even though an ambient temperature is not so low or the apparatus is restarted immediately after a temporary machine stop, the energization of the heater is forcibly carried out for a predetermined period as routine, which delays the commencement of the printing operation and lowers the machine efficiency.